Lumpy's Lemon Drop with Pre-gain

[induction]

Voltages on the FET look fine. What about on the BJT?

[tracer03]

induction wrote

Voltages on the FET look fine. What about on the BJT?

There’s a problem:
OG, cbe:
6.71
.780 mV
217.1 mV

Mine, cbe:
.5 mV
1.6mV
0 (-.8) mV

[induction]

You appear to have two cuts in the wrong place, according to the layout. They are on the third and fourth row from the bottom, on the left hand side of the picture of the copper side. The top one doesn't matter, but the bottom one discconnects the 22n coupling cap coming from the bjt collector so that the signal never reaches the fet and connects to pregain 3 instead.

This fact does not explain the voltages on the bjt.

This layout does not appear to match any of the ones you've posted in the thread. It's closest to the second from last one, but I can't figure out which wires are attached on the lower, right hand, component side. Can you post the layout that matches what you built?

[tracer03]

induction wrote

You appear to have two cuts in the wrong place...

SONOFA...I honestly don't know what kind of mental malfunction I had going on there. Cuts in the wrong place, board all wrong...the social distancing is getting to me. Will work on a corrected layout and build today. (thanks for the catch)

[tracer03]

So, I corrected the layout, and rebuilt last night. I get sound, etc. - but it only reaches unity gain with both the pregain and volume dimed. I expected the pregain to kill the signal when turned all the way down, but with the drive at 0 the signal is also killed.

The only thing I *didn't* do was a 10k resistor off lug 1 of the volume pot - I took it via lead to ground when I hooked it up via my test box.

I also tried turning the trim pot, but it didn't seem to have an impact on the voltages. I omitted the 10k resistor that came off the top rail to the LED connection (since it didn't connect anywhere in the circuit path) so I'm pretty sure that wouldn't make a difference.

Voltages:
Q1
C: 6.32
B: .897
E: .286

Q2:
D: 2.737
S: .674
G: 0

Incorrect Layout removed

[induction]

tracer03 wrote

So, I corrected the layout, and rebuilt last night. I get sound, etc. - but it only reaches unity gain with both the pregain and volume dimed. I expected the pregain to kill the signal when turned all the way down, but with the drive at 0 the signal is also killed.

Neither of those should kill the signal. The minimum gain of the Q1 stage is 10x.

The only thing I *didn't* do was a 10k resistor off lug 1 of the volume pot - I took it via lead to ground when I hooked it up via my test box.

The 10k is supposed to go between ground and lug 3 of the Drive pot. This explains why you get no signal at Drive=0. While the pregain control actually controls the gain of the Q1 stage, the Drive pot is just a volume control between Q1 and Q2. Without the 10k, you are grounding the input of Q2 at Drive=0.

I also tried turning the trim pot, but it didn't seem to have an impact on the voltages.

I think you have lug1 of the trimmer on 9V and lug 3 on the FET drain. You need lug 2 on the FET drain. You can add a jumper between rows 2 and 3 just to the left of the trimmer. You can leave the other jumper in place. It won't hurt anything.

Voltages:
Q1
C: 6.32
B: .897
E: .286

Q2:
D: 2.737
S: .674
G: 0

Q1 voltages look good. Q2 voltages will probably be fine after you install the jumper and tune the trimmer.

Corrected layout:

The second cut from the right needs to move up 1 row. I suspect the error is only on the layout and not on your build. You would get no output at all with the cut where it is on the layout since it separates Drive 2 from the FET gate.

[tracer03]

induction wrote

The second cut from the right needs to move up 1 row. I suspect the error is only on the layout and not on your build. You would get no output at all with the cut where it is on the layout since it separates Drive 2 from the FET gate.

Row 2, col 14, correct?

induction wrote

The 10k is supposed to go between ground and lug 3 of the Drive pot. This explains why you get no signal at Drive=0. While the pregain control actually controls the gain of the Q1 stage, the Drive pot is just a volume control between Q1 and Q2. Without the 10k, you are grounding the input of Q2 at Drive=0.

So for that, I can just put a standing resistor in the 3rd col from the right, rows 8 & 9? At least to get it working...

induction wrote

I think you have lug1 of the trimmer on 9V and lug 3 on the FET drain. You need lug 2 on the FET drain. You can add a jumper between rows 2 and 3 just to the left of the trimmer. You can leave the other jumper in place. It won't hurt anything.

 
The specs for the trimpot shows lug 1 is in row 1, 2 in 2, 3 in 3. To correct the layout, would I want to rotate the trimmer 180 degrees and adjust the link?

induction wrote

The second cut from the right needs to move up 1 row. I suspect the error is only on the layout and not on your build. You would get no output at all with the cut where it is on the layout since it separates Drive 2 from the FET gate.

Which row? If 2nd from top - that cut is also on the existing board.

[induction]

tracer03 wrote

induction wrote

The second cut from the right needs to move up 1 row. I suspect the error is only on the layout and not on your build. You would get no output at all with the cut where it is on the layout since it separates Drive 2 from the FET gate.

Row 2, col 14, correct?

No, Row 6, col 18.

induction wrote

The 10k is supposed to go between ground and lug 3 of the Drive pot. This explains why you get no signal at Drive=0. While the pregain control actually controls the gain of the Q1 stage, the Drive pot is just a volume control between Q1 and Q2. Without the 10k, you are grounding the input of Q2 at Drive=0.

So for that, I can just put a standing resistor in the 3rd col from the right, rows 8 & 9? At least to get it working...

I'm sorry. I meant lug 1 of the Drive pot, not lug 3. I don't see an obvious place to put the resistor on the board. You could put it between lug 1 and the pot body like the production unit, as long as the pot body is grounded. But really, you don't need it if you're ok with silence at Drive=0. It's not needed just to get the unit working. You can leave lug 1 grounded and it will work just fine.

induction wrote

I think you have lug1 of the trimmer on 9V and lug 3 on the FET drain. You need lug 2 on the FET drain. You can add a jumper between rows 2 and 3 just to the left of the trimmer. You can leave the other jumper in place. It won't hurt anything.

 
The specs for the trimpot shows lug 1 is in row 1, 2 in 2, 3 in 3. To correct the layout, would I want to rotate the trimmer 180 degrees and adjust the link?

I would just put a link between rows 2 and 3, column 15, and leave the existing link in place. That's a totally appropriate way to do it because it jumpers the wiper to the unused lug, which I always do just in case the wiper fails. There's no way to rotate the trimmer to make it simpler. You'd have to add an additional cut.

[tracer03]

induction wrote

--fixes --

Okay, modified the layout based on your fix directions, with the exception of the 10k resistor to lug 1 of the drive pot. I'll throw it together later and report back. If you see any errors please let me know.

Thanks again for all your help!

[induction]

Looks good to me

[tracer03]

Well, it works! Built it, tweaked the trimmer and it sounds great. I'd still like to get a cleaner layout, and avoid wiring a resistor to the Drive pot so the signal doesn't die with the drive down - so if anyone wants to take a stab at cleaning up my mess...

Induction, thanks again for your help!

[induction]

Cool! Glad you got it working, and happy to help.

[HamishR]

Hey Tracer - as requested I had another look at this and this is what I came up with.  I haven't tested it and maybe I will - I liked the original Lemon Drop a lot.  It's a bit of a bugger to layout cleanly. You can leave more space around if you want to - maybe not such a bad idea. I don't like having Volume 3 so close to the Tone (and therefore Input) leads so I hope it doesn't cause any issues. But it's compact!

[HamishR]

Well I had some time this afternoon so I built it. You will see I changed the 5K trimmer to 20K because i couldn't get the Jfet biased otherwise. I'm using an old metal case BC108C for Q1 and I have over 6 1/2 VDC on the Collector so I'll have to fix that too.

As it is it sounds pretty good if I leave Pregain low. If I raise Pregain the gain does rise but it doesn't sound very good. Kinda sounds like an over-biased tranny! Tomorrow I'll raise the value of that 22K and see if I can get it to bias around 4.5V. The EQ sounds good but the break-up is nasty.

So far no issues with the layout. It works, just needs biasing.

[HamishR]

Well it's tomorrow morning now.  :-)

Last night I found that a 36K resistor biased the BC108 about right, so today I fired it up.  Sounds a lot better now, but I still prefer it with Pregain low. If you raise Pregain the distortion starts to take on the character of a Colorsound overdriver - that harsh, gritty, overtly transistor kind of distortion.

So the layout I posted is verified - you just need to tweak the value of the 22K resistor to suit your transistor.

I think I like the 3-knob version better.

[tracer03]

HamishR wrote

Well it's tomorrow morning now.  :-)

Last night I found that a 36K resistor biased the BC108 about right, so today I fired it up.  Sounds a lot better now, but I still prefer it with Pregain low. If you raise Pregain the distortion starts to take on the character of a Colorsound overdriver - that harsh, gritty, overtly transistor kind of distortion.

So the layout I posted is verified - you just need to tweak the value of the 22K resistor to suit your transistor.

I think I like the 3-knob version better.

I think that biasing Q1 is crucial: I've built a couple of 3-knobber Lemon Drops, and they all seemed to be...'too much'.  Maybe it was the Gain control: didn't seem to have a gradual sweep, or much variance across the whole range. Did you measure the BC108? Wondering if the gain would have an impact.

[HamishR]

The 36K gives me 4.59VDC on the collector. That tallies well with my notes on the previous 3-knob Lemon Drop where it was stated that 4.6VDC is ideal. I didn't measure the transistor before installing.  Probably should have.